One common type of packaging for the above types of liquid or pourable food products is a container made from a blank or a continuous web of laminated packaging material constituted by a layer of fibre-based material such as paper, which is covered on each side with a plastics material such as polyethylene. In order to manufacture aseptic packages, one face of the polyethylene coated paperboard is coated with a barrier material such as aluminum or a synthetic barrier material, which in turn is coated with a plastics material. For manufacturing aseptic packages, the web of packaging material passes through an aseptic chamber and is sterilized, for example by means of the application of a sterilizing agent such as hydrogen peroxide which is successively evaporated by heating and/or by irradiating the packaging material with light of appropriate wavelength and intensity. The sterilized web of laminated packaging material is bent and longitudinally sealed to form a packaging material tube. The packaging material tube in practice forms an extension of the aseptic chamber and is continuously filled with sterile or sterile treated liquid food product.
The longitudinally sealed tube filled with liquid food product is then clamped between pairs of forming and sealing members which transversely seal and form the product filled tube to form pillow-shaped packages. The forming and sealing members may be mounted in mutually facing pairs, on respective pairs of continuously-moving mutually-facing chains having opposite directions of rotation, whereby to form and seal the packages therebetween. One known type of machine of this kind is disclosed in U.S. Pat. No. 3,300,944. Alternatively, the forming and sealing members may be mounted on pairs of reciprocating jaws. One example of this kind of machine is the TBA/19 machine manufactured by Tetra Brik Packaging Systems of Via Delfini 1, 41100 Modena, Italy. The filled and sealed pillow-shaped packages are then transported to a final folding station, wherein the pillow-shaped packages are mechanically folded into a final shape, such as a parallelepiped shape. One example of commercially available packages of this type are the parallelepiped packages commonly known by the registered trademark "Tetra Brik Aseptic".
While the above mentioned types of machine are excellent from many standpoints, they have been found to be susceptible to improvements with respect to the mechanism for filling the tube of laminated packaging material with product. Different solutions have been proposed for solving the technical problem of how to control of the level of product in the packaging material tube, to ensure correct functioning of the machine, even at high speeds and also with an ever-increasing range of products of varying characteristics, such as viscosity, susceptibility to foaming etc.
Known from U.S. Pat. No. 3,470,672 is an apparatus for packaging viscous liquid materials which includes means for forming a longitudinally-sealed continuous tubular container from a web of packaging material, a filler pipe disposed axially of the tubular container for introducing liquid material therein, and members for successively clamping the tube at regularly spaced intervals for forming and transversely sealing packages filled with product. A flexible bell-shaped member of resilient flexible material is secured about the filling pipe, and the circumferential skirt of the bell-shaped member slidingly and sealingly engages the inner wall of the of the tubular container to form a closed space defined by the inside of the bell-shaped member and the level of the viscous liquid above the bottom edge of the skirt. The thickness of the bell-shaped member diminishes towards the lower edge thereof, to improve the sliding and sealing engagement with the inner wall of the tubular container. Means responsive to pressure are provided in the closed space for controlling the rate of introduction on viscous liquid through the filler pipe. The flexible bell-shaped member is provided with vents to permit the escape of air bubbles introduced with the viscous liquid into the closed space. While this system provided a greater degree of control of viscous liquids, it could not be used to facilitate product level control in modern high-speed liquid food packaging machines. In fact the requirement for sliding sealing engagement between the inside of the packaging material tube and the bell-shaped member, would prejudice the integrity of the packaging material, cause excessive wear of the bell-shaped member and imply a risk of contaminating the liquid food product with residues of the material constituting the bell-shaped member through friction generated at the inner wall of the packaging material tube.
Another known type of product level control mechanism includes a product filling pipe having an end portion located axially within the packaging material tube. The end portion of the filling pipe has a downwardly open end which is immersed in the product contained in the packaging material tube. A float member is slideably connected to the filling pipe and is in turn connected to a butterfly valve located within the filling pipe. The float member is connected to the butterfly valve by a kinematic mechanism, which causes the butterfly valve to throttle the filling pipe when the float rises above a predetermined level and opens the valve when the float falls below such level. Thus, every time that the longitudinally sealed tube of packaging material is clamped by the forming and sealing members, the product level rises in the packaging material tube, thereby causing the float to rise and the butterfly valve to throttle the filling pipe, thereby slowing down the product flow into the packaging material tube. The continuous movement of the packaging material tube then causes the product level to drop, thereby causing the level of the float member to fall, with consequent opening of the butterfly valve for increasing the rate of filling of the product tube, ready for being clamped again by the forming and sealing members. This cycle is repeated continuously during the normal operation of the machine and the level of the product within the packaging material tube oscillates continuously.
As the speed of the machines for packaging pourable food products has increased, so has the range of products which these machines are used for packaging. There is thus a wider range of viscosities to cope with, as well as a wider range of degrees of susceptibility to foaming. This adds to the problem of how to control the product level in the packaging material tube. The product level must be controlled in a manner which has sufficiently rapid response times to cope with the rapidly fluctuating product level and with products of different characteristics. Furthermore, such systems must withstand high working temperatures, and be capable of being cleaned and disinfected with chemical agents at high temperatures.
Flow-control valves, located on the product line in a position remote from the end portion of the filling pipe, have been used as an alternative to the above-cited butterfly valves for controlling the rate of flow of the product through the filling pipe. The flow-control valve may be controlled by one or more sensors. The known sensors are either located externally of the packaging material tube for detecting the position of a detectable float which indicates the product level within the packaging material tube, or on the filling pipe for directly contacting and thereby detecting the presence of product within the longitudinally sealed packaging material tube at a predetermined level. In this case, each time that the sensor either directly contacts the product at a predetermined location on the filling pipe, or each time that the position of a detectable float is detected at a predetermined product level, a signal is generated which indicates that the required product level has been reached for permitting correct functioning of the liquid food packaging machine and the valve automatically controls the flow of product through the filling pipe and into the packaging material tube. The flow rate varies continuously. Each time that the packaging material tube is gripped between the pairs of package forming and sealing members and the product level rises in the tube, the sensor-controlled valve automatically throttles the flow of product through the filling pipe to reduce the product flow rate. The continuous movement of the packaging material tube then causes the product level to drop, and when the sensor is no longer contacted by the product or no longer senses the float at the predetermined position, a signal is generated which causes the valve to automatically open and increases the product flow rate.
Although these latter systems afford a smoother control of the product flow when compared to the butterfly valves, problems have been encountered when using these filling systems, particularly in chain-type machines (as disclosed e.g., in the above-mentioned U.S. Pat. No. 3,300,944) for packaging liquid food products, and especially when such chain-type machines operate at high-speed.
It has been found that when operating at very high speeds, i.e., when operating liquid food packaging machines at a production rate of 18000-24000 packages per hour, additional problems arise relating to the control of the product level in the packaging material tube, due to the fact that between successive package filling, forming and sealing operations performed by the forming and sealing members, there is not sufficient time for the product level to fall significantly by gravity. Low pressures are in fact generated within the packaging material tube during the package forming cycle, caused by the dynamics of the liquid food product inside the tube. Such low pressures can adversely affect the formation of the continuous longitudinally sealed tube with the packaging material web.